I. INTRODUCTORY 
Definitions 
A tidal datum plane is a plane of reference for elevations, determined from the rise 
and fall of the tides. Various tidal planes may be derived, and each is designated by a 
definite name, as, for example, the plane of mean high water, the plane of half-tide 
level, the plane of lower low water. 
The tide is the name given to the alternate rising and falling of the level of the sea, 
which at most places occurs twice daily. The striking feature of the tide is its intimate 
relation to the movement of the moon. High water and low water at any given place 
follow the moon’s meridian passage by a very nearly constant interval; and since the 
moon in its apparent movement around the earth crosses the meridian at any place 50 
minutes later each day on the average, the tide at most places likewise comes later each 
day by 50 minutes on the average. 
With respect to the tide, the ‘‘moon’s meridian passage”’ has a special significance. 
It refers not only to the instant when the moon is directly above the meridian but also 
to the instant when the moon is directly below the meridian, or 180° distant in longitude. 
In this sense there are two meridian passages in a tidal day, and they are distinguished 
“by being referred to as the upper and lower meridian passages or upper and lower 
transits. 
The interval between the moon’s meridian passage (upper or lower) and the follow- 
ing high water is known as the “high-water lunitidal interval.’’ Likewise, the interval 
between the moon’s meridian passage and the following low water is known as the 
“low-water lunitidal interval.’’ For short they are called, respectively, high-water 
interval and low-water interval, and abbreviated as follows: HWI and LWI. 
With respect to the rise and fall of the water due to the tide, high water and low 
water have precise meanings. They refer not so much to the height of the water as to 
the phase of the tide. High water is the maximum height reached by each rising tide 
and low water the minimum height reached by each falling tide. 
It is important to note that it is not the absolute height of the water which is in 
question, for it is not at all infrequent at many places to have the low water of one day 
higher than the high water of another day. Whatever the height of the water, when 
the rise of the tide ceases and the fall is about to begin, the tide is at high water, and 
when the fall of the tide ceases and the rise is about to begin the tide is at low water. 
The abbreviations HW and LW are frequently used to designate high and low water, 
respectively. 
The difference in height between a high water and a preceding or succeeding low 
water is known as the range of the tide or range. Since the heights of high and low 
water at any place vary from day to day, it follows that the range of tide likewise 
varies from day to day. It is, in fact, this variation which gives rise to the problems 
involved in the determination of tidal datum planes. 
